The present invention relates to a FSK demodulator for a frequency-modulation modem.
It is recalled that there is currently designated under the name modems, modulators-demodulators which comprise:
a modulator which converts a random succession of binary signals equal to 0 or 1 into a periodic analog signal, modulated in frequency, in amplitude or in phase, and transmittable by means of the telephone network; PA1 a demodulator which converts the modulated analog signal supplied by the network into a succession of 0 or 1. PA1 two transverse parallel filters which comprise a shift register with n+1 stages and which form the convolution between the input signal of the sampled filter and the pulse response of the filter defined by the weighting coefficients h.sub.k. The weighting coefficients h.sub.k of the two filters are functions of the product .omega.p.tau., a given function called f.sub.R (.omega.p.tau.) being the weighting coefficient of one of the filters and another given function called f.sub.I (.omega.p.tau.) being the weighting coefficient of the other filter, with: PA1 a device for squaring or taking the modulus of the signal coming from each filter. PA1 its simplicity of construction; PA1 its great ease of integration particularly in the case where the filters are charge-transfer filters; PA1 its high performances.
The present invention relates to frequency-modulation modems which are known under the English name of Frequency Shift Keying or FSK.
In FSK modems, the modulator causes a sine curve at frequency F.sub.A to correspond to a zero and the modulator causes a sine curve of frequency F.sub.Z to correspond to a one.
The specifications of the CCITT lay down all the characteristics of the modems, thus a 1200-baud modem has been studied for which F.sub.A =2100 Hz and F.sub.Z =1300 Hz. This modem operates in duplex (i.e., using simultaneously the same line) with another 75-baud modem for which F.sub.A =450 Hz and F.sub.Z =390 Hz.
The 1200-baud modem will be cited by way of example in what follows.
In the prior art demodulators for frequency-modulation modems have first a chopper for the sinusoidal signal received from the telephone line followed by a shift register which imposes a delay .tau. and a device which ensures multiplication between the signal coming directly from the chopper and this same signal delayed by .tau..
The delay .tau. is chosen so that: .omega..sub.c .multidot..tau.=.pi./2 with .omega..sub.c defined as follows: .omega..sub.c =2.pi..multidot.F.sub.C with f.sub.o =(F.sub.A -F.sub.Z)/2 and F.sub.C =F.sub.A -f.sub.o.
Thus, at the output of the multiplication device, the signals at F.sub.A and F.sub.Z are superimposed on DC voltages which are equal but of opposite signs and it is easy to detect them and to transform the analog signal received from the line into a succession of 0 and 1.
For F.sub.A =2100 Hz and F.sub.Z =1300 Hz, the frequency F.sub.C equals 1700 Hz and .tau. equals 150 .mu.s.